1. Field
Exemplary embodiments of the present invention relate to a normally off transistor and a method of fabricating the same, and more particularly, to a normally off nitride-based transistor using a nitride-based semiconductor and a method of fabricating the same.
2. Discussion of the Background
Recently, AlGaN/GaN HEMT (High Electron Mobility Transistor), which is one of GaN-based electronic devices, has power density of about 4 W/mm to 8 W/mm, and maximum output power of a single chip is about 90 W in 3 GHz. Since the GaN electronic device has high power density and efficiency as compared with the existing GaAs electronic device, it is advantageous in that it is possible to reduce a module size to about 50% or more and achieve lightness.
However, since the GaN electronic device is very expensive and a mass production process of a high output device of 25 W or more has not been completed, it is difficult to satisfy a large order. In order to lower the cost of the GaN electronic device, when an AlGaN/GaN stack structure is formed in an inexpensive substrate such as a sapphire substrate or a silicon substrate instead of an expensive GaN substrate, since threading dislocation density is high, it is not reliable.
A normally off characteristic is required for a GaN transistor device in relation to a switching characteristic, and when a conventional horizontal AlGaN/GaN HEMT has the normally off characteristic, since a threshold voltage characteristic is degraded or process control is difficult, reproducibility and reliability are reduced. When the threshold voltage characteristic is degraded, an electronic device may abnormally operate in a high temperature operation or a noise environment.
For example, in a recess gate type horizontal normally off GaN HEMT, the thickness of an AlGaN layer below a trench, in which a recess gate is installed, should be controlled to about several nm (for example, 1 nm to 5 nm) in an etch process, but it is difficult to strictly manage an etch depth in such an etch process, resulting in the deterioration of productivity.
In another example, in a p-GaN gate type horizontal normally off GaN HEMT, a p-GaN layer is disposed under a gate electrode, so that a potential well of a valence band existing around an interface of AlGaN/GaN heterojunction layers under the gate electrode moves above the Fermi level, and thus a discontinuous region with no two dimensional electron gas (2DEG) is formed in a 2DEG channel, resulting in the achievement of a normally off characteristic. However, since a threshold voltage of the conventional GaN HEMT is reduced due to the characteristic of the p-GaN layer itself, there is a limitation in utilizing the normally off characteristic.
In a dry etch process for forming a p-GaN gate, since positive charge is accumulated on the surface of the AlGaN layer due to plasma damage, a current collapse phenomenon is accelerated, so that a 2DEG characteristic is degraded.
In addition, the aforementioned recess gate type or p-GaN gate type horizontal normally off GaN HEMT has the following problems due to electric field concentration at an edge of the gate electrode and high threading dislocation density.
First, when a plurality of pits or cracks exist in the AlGaN/GaN layer, mechanical stress is easily accumulated in the AlGaN layer.
Then, in an AlGaN barrier layer for forming the AlGaN/GaN heterojunction, the AlGaN barrier layer is damaged step by step due to an inverse Piezoelectric effect in which electrical energy is converted into mechanical energy through a permanent dipole generated by asymmetry of crystallization. In this case, there is a problem that maximum current, which may flow through the device, is reduced and leakage increases.
Last, as the strength of voltage applied to the normally off GaN HEMT is increased or the accumulation of use time, a part having dislocation or defect is quickly expanded, so that the performance of the device easily deteriorates. Particularly, when the threading dislocation density increases to about 108/cm2 to about 1010/cm2, since an on-resistance rise rate is high in a high temperature operation, breakdown voltage is lowered and leakage increases.